Diatomaceous earth products may be obtained from diatomaceous earth (also called “DE” or “diatomite”), which is generally known as a sediment-enriched in biogenic silica (i.e., silica produced or brought about by living organisms) in the form of siliceous skeletons (frustules) of diatoms. Diatoms are a diverse array of microscopic, single-celled, golden-brown algae generally of the class Bacillariophyceae that possess an ornate siliceous skeleton of varied and intricate structures including two valves that, in the living diatom, fit together much like a pill box.
Diatomaceous earth may form from the remains of water-borne diatoms and, therefore, diatomaceous earth deposits may be found close to either current or former bodies of water. Those deposits are generally divided into two categories based on source: freshwater and saltwater. Freshwater diatomaceous earth is generally mined from dry lakebeds and may be characterized as having a low crystalline silica content and a high iron content. In contrast, saltwater diatomaceous earth is generally extracted from oceanic areas and may be characterized as having a high crystalline silica content and a low iron content.
In the field of fluid filtration, diatomaceous earth may be employed as a filter aid, and methods of particle separation from fluids may employ diatomaceous earth products as filter aids. The intricate and porous structure unique to diatomaceous earth may, in some instances, be effective for the physical entrapment of particles in filtration processes. It is known to employ diatomaceous earth products to improve the clarity of fluids that exhibit “turbidity” or contain suspended particles or particulate matter. “Turbidity” is the cloudiness or haziness of a fluid, where the haze may be caused by individual particles that are suspended in the fluid. Materials that may cause a fluid to be turbid include, for example, clay, silt, organic matter, inorganic matter, and microscopic organisms.
When diatomaceous earth products are used, for example, as filter aids, it may be desirable to heat treat the natural diatomaceous earth in order to reduce the amount of naturally occurring organics and/or volatiles remaining in the natural diatomaceous earth. However, heat treatment may result in an increase of the cristobalite form of the silica present in naturally occurring diatomaceous earth, particularly when the diatomaceous earth is heat treated at temperatures above, for example, 1000° C. The presence of a significant amount of cristobalite is generally undesirable in uses such as filter aids due to its potentially unhealthy properties in higher concentrations. Thus, it may be desirable to provide compositions and methods for heat treating diatomaceous earth that do not result in significant formation of cristobalite in the diatomaceous earth product.
In addition, when diatomaceous earth products are used, for example, as filter aids, it may be desirable to reduce the level of soluble metal present. As used herein, the term “soluble metal” refers to any metal that may be dissolved in at least one liquid. Soluble metals may include, but are not limited to, iron, aluminum, calcium, vanadium, chromium, copper, zinc, nickel, cadmium, and mercury. For example, when a filter aid including a diatomaceous earth product used to filter at least one liquid, at least one soluble metal may dissociate from the diatomaceous earth filter aid and enter the liquid. In many applications, such an increase in metal content of the liquid may be undesirable and/or unacceptable. For example, when a filter aid including diatomaceous earth is used to filter beer, a high level of iron dissolved in the beer from the filter aid may adversely affect sensory or other properties, including but not limited to, taste and shelf-life. Thus, it may be desirable to provide compositions and methods for heat treating diatomaceous earth that do not result in significant amounts of soluble metal in the diatomaceous earth product.
In many filtration applications, a filtration device may include a filter element, such as a septum, and a filter-aid material. The filter element may be of any form such that it may support a filter-aid material. For example, the filter element may include a cylindrical tube or wafer-like structure covered with a plastic or metal fabric of sufficiently fine weave. The filter element may be a porous structure with a filter element void to allow material of a certain size to pass through the filtration device. The filter-aid material may include one or more filtration components, which, for example, may be inorganic powders or organic fibrous materials. Such a filter-aid material may be used in combination with a filter element to enhance filtration performance.
For example, the filter-aid material may initially be applied to a septum of a filter element in a process known as “pre-coating.” Pre-coating may generally involve mixing a slurry of water and filter-aid material, and introducing the slurry into a stream flowing through the septum. During this process, a thin layer, such as, for example, a layer of about 1.5 mm to about 3.0 mm thickness, of filter-aid material may eventually be deposited on the septum, thus forming the filtration device.
During filtration of a fluid, various insoluble particles in the fluid may become trapped by the filter-aid material. The combined layers of filter-aid material and particles and/or constituents to be removed accumulate on the surface of the septum. Those combined layers are known as “filter cake.” As more particles and/or constituents are deposited on the filter cake, the filter cake may become saturated with debris to the point where fluid is no longer able to pass through the septum.
To combat this situation, a process known as “body feeding” may be used. Body feeding is the process of introducing additional filter-aid material into the fluid to be filtered before the fluid reaches the filter cake. The filter-aid material will follow the path of the unfiltered fluid and will eventually reach the filter cake. Upon reaching the filter cake, the added filter-aid material will bind to the cake in a similar manner to how the filter-aid material is bound to the septum during the pre-coating process. The additional layer of filter-aid material may cause the filter cake to swell and thicken, and may increase the capacity of the filter cake to entrap additional debris. The filter aid typically has an open porous structure, which maintains an open structure in the filter cake, thus ensuring continued permeability of the filter cake.